INTRODUCTION TO THE HIGHER INVERTEBRATES 237 



99. Evolutionary Relationships of the Coelenterates 



Soon after the concept of evolution was accepted, it was recognized 

 that the gastrula stage of higher animals may indicate or hark back to 

 an ancestral form, whose basic body plan was similar to that found in 

 adult coelenterates today. It is primarily for this reason that the coel- 

 enterates were placed close to the base of the stock that gave rise to 

 higher animals. Continuing this approach, it was further recognized that 

 the blastula may represent a still earlier form, an organism that was 

 essentially a hollow ball of cells. Volvox (Fig. 8.6) is a protozoan colony 

 with such a form, and Volvox is often used as an example to represent 

 the form of the metazoan ancestor. The various degrees of colony forma- 

 tion in the Phytomonadina {Chlamydomonas, Gonium, etc.) can be used 

 to show how the blastula-like stage evolved from simple protozoa, and it 

 then remains only to make a gastrula out of a blastula. 



Although the various phytomonads form an excellent series to show 

 how colonial forms such as Volvox may have evolved, it can hardly be 

 concluded that this group of flagellates is particularly close to the stock 

 that actually gave rise to the metazoa. The phytomonads are strictly 

 autotrophic and they are also haploid (as described in Chapter 8). Fur- 

 thermore, all of the present colonial phytomonads are fresh-water organ- 

 isms, and it seems likely that the metazoa have had a primarily marine 

 origin and evolution. Hence, the use of these organisms as an example 

 of a possible series of stages in the origin of the metazoa should not be 

 confused with the proposal that they represent living descendants of the 

 actual ancestors. 



The coelenterates and ctenophores are usually considered to be 

 the simplest metazoa other than sponges. They lack mesodermal tissues 

 and excretory organs. To a considerable extent they can be regarded as 

 organisms made of two layers, folded and warped in various ways. Most 

 students believe that these groups are primitively simple, but a few prefer 

 the possibility that they once had a bulky mesoderm that has been lost. 



100. The Evolution of Three Germ Layers 



The flatworms appear to lie close to the stock that produced all of 

 the remaining phyla. They are relatively simple in the sense that the gut 

 lacks a separate mouth and anus and neither a body cavity nor a cir- 

 culatory system is present. They are more complex than the preceding, 

 however, in that all three germ layers (ectoderm, mesoderm, endoderm) 

 are present and well defined. They have protonephridia, and they have 

 muscle layers added to the body wall. These latter characters link the 

 remaining phyla, suggesting strongly that from the flatworms up, at 

 least, all metazoa have a common origin. 



The nemertean body plan can be derived from the flatworm type 

 by the addition of the proboscis and a circulatory system, a separation 

 of the mouth and anus, and minor elaborations of other structures. The 

 resemblance of the nemertean epidermis and sense organs to those of 

 the flatworms is very striking. 



